1. Field of the Invention
The present invention relates to a method for operating an electric press which comprises an electrically actuated press ram, at least one displacement sensor for sensing positions of the displacement of the press ram during its working stroke, at least one force sensor for sensing compressive force applied by the press ram during the working stroke onto workpieces to be processed, and a control system which controls the working stroke in terms of displacement and compressive force, having the steps:
a) moving the press ram into its initial position; PA1 b) lowering the press ram onto the workpieces to be processed, and measuring the compressive force; PA1 c) detecting the onset of pressing based on a rise in the compressive force; PA1 d) further lowering the press ram to perform the pressing operation, and monitoring the compressive force being applied; and PA1 e) halting the press ram when the latter has reached a preset joining or end position.
2. Related Prior Art
A method of this kind is known from U.S. Pat. No. 5,483,874.
The known method is carried out on an electric press which comprises a spindle drive driven by an electric motor. The threaded spindle of the spindle drive is mounted rotatably but axially nondisplaceably, while the spindle nut is mounted nonrotatably but axially displaceably, and is joined to the press ram.
The electric press has displacement sensors and force sensors in order to sense the profile of the compressive force versus the displacement of the press ram during its working stroke and report it to a control system which, on the basis of these data, controls the working stroke.
At the beginning of a pressing operation, the press ram is moved into its initial position above the workpiece, and is then lowered onto the workpiece or workpieces to be processed.
The compressive force is measured during this lowering, and the fact that the pressing operation is beginning is detected on the basis of a rise in this compressive force, whereupon the lowering rate of the press ram is decreased.
The press ram is lowered further during the pressing operation which then follows, the applied compressive force then being monitored to determine whether it remains constant during the pressing operation. The displacement of the press ram also continues to be monitored in order to detect when it has reached a joining position (therein called the end position) in which the press ram essentially does not move any farther down. When this end position has been reached, the press ram is retracted.
If the end position is not reached, or if a constant pressure is not applied during the pressing operation, the pressing operation is terminated.
It has now been found that it is possible, in the context of this kind of method, for a compressive force that it is sometimes too high and also sometimes too low to be applied, depending on tolerances of the workpieces to be processed, so that some of the workpieces are not correctly joined and some are damaged by excessive compressive force.
In order to process the workpieces gently and reproducibly, they therefore must have very narrow tolerances; if these tolerances are exceeded upward or downward, the known method terminates the pressing operation because the end position is not reached and/or the compressive force is not constant; this can result in unnecessary wastage.